1. Field of the Invention
The present invention relates to organic electroluminescent (EL) displays, and more specifically, to a multi-color EL display array panel and a method for fabricating the same.
2. Discussion of the Related Art
Organic EL devices, also called organic light emitting diodes (LEDs), are becoming very popular because of their possible application to flat panel displays (FPDs). They are extremely thin, matrix-addressable and can be operated at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional characteristics suitable for next generation FPDs such as little dependence on viewing angle and good device-formability on flexible substrates among other things. Organic LEDs differ fundamentally from conventional inorganic LEDs. While the charge transfer in inorganics is band-like in nature and the electron-hole recombination results in the interband emission of light, organic films are generally characterized by the low-mobility activated hopping transport and the emission is excitonic. Organic EL devices are also substantially different from conventional inorganic EL devices, especially in that organic EL devices can be operated at low DC voltages.
A substantial amount of research has been directed toward the efficiency improvement and color control of organic LEDs. The efficiency of some organic EL devices has now been demonstrated to be close to its theoretical limit and certainly adequate for many commercial applications. Moreover, the color control is probably not limiting for many potential applications such as monochromatic and simple multi-color displays. However, it would be more difficult to generate complex multi-color and full color displays. There are several ways of full color generation. First, one may form pixels of red(R), green (G), blue(B) by separately depositing the light emitting layer of each pixel. Secondly, one can generate R, G & B by passing white light through a set of R,G,B color filters. Finally, one can obtain R, G & B by passing blue light through color changing media as described in U.S. Pat. No. 5,126,214. A similar scheme has been devised by C. Tang et al. in U.S. Pat. No. 5,294,870, and the device structure suggested therein is described in FIG. 1.
Referring now to FIG. 1, illustrated therein is a cross-sectional side view of several sub-pixels in a multi-color EL display panel. This background art multi-color EL display array panel may be fabricated by the following process. An array of patterned fluorescent medium G and another medium R (2 in FIG. 1) are formed on top of a transparent support 1. Following the formation of first electrode stripes 3 on a planarizing layer 7, an array of parallel walls 6 has been prepared to laterally separate each sub-pixel element. Then the organic EL layer 4 is formed using a conventional deposition technique. To insure lateral separation of each sub-pixel, next, the deposition of second electrode elements 5 is performed in such a manner that the direction of traveling metallic vapor forms an angle .theta. with the normal of the deposition surface. The use of conventional pixellation technique is precluded due to the nature of organic materials which are extremely vulnerable to the damage by most solvents. That is why they resorted to a rather complicate patterning method utilizing a shadowing effect. The idea of separating each stripe of the second electrode by the deposition at an oblique angle may be conceivable, but is not suitable at all for mass productions. First of all, it is impossible to have a vapor source which may set the direction of traveling metallic vapor at a constant angle .theta. with the surface normal over the whole panel surface. Whether one has either a point source or a large area source, the angle .theta. would vary with the position of pixel on the transparent support 1 because of the diverging nature of vapor. One may be able to obtain a certain degree of separation with a complicate arrangement of source and support, but only at the sacrifice of production yield. Finally, a protective film 8 is formed, as seen in FIG. 1.
In summary, the background art described above has the following problems.
First, R,G,B from adjacent sub-pixels can be easily mixed, resulting in the color contamination by neighboring sub-pixels because of having no optical barriers between the fluorescent media 2.
Secondly, the suggested method to separate the electroluminescent layer and the second electrode element of a sub-pixel from those of another sub-pixel is not suitable for mass productions.